Stabilization of Amine-Containing CO2 Adsorbents and Related Systems and Methods

ABSTRACT

The present invention provides (i) a method and system for stabilizing the performance of amine-containing CO 2  adsorbents using wet feed gas and/or wet purge gas and (ii) a method for regeneration of deactivated amine-containing CO 2  adsorbents via hydrolysis of the urea groups formed during deactivation.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/976,557, filed Dec. 22, 2010, and claims priority to and the benefitof U.S. Provisional Patent Application No. 61/289,198, filed Dec. 22,2009, entitled “Stabilized Amine-Containing CO₂ Adsorbents and Relatedsystems and Methods,” which is incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

The present invention pertains to the field of adsorption methods andsystems for carbon dioxide capture, more particularly, to the field of(i) adsorption methods and systems, (ii) stabilization methodologies ofamine-containing adsorbents for CO₂ separation and the purification ofcarbon dioxide-containing gases, and (iii) regeneration of deactivatedamine-containing CO₂ adsorbents

BACKGROUND

Capture of anthropogenic carbon dioxide from large sources of emissionsuch as fossil fuel power plants is a key target in the ongoing effortto mitigate the effect of greenhouse gases on global climate change.Although mature, the liquid phase amine scrubbing technology suffersfrom inherently high regeneration cost, equipment corrosion and amineoxidative degradation. As a result, there is a strong tendency todevelop recyclable solid sorbents to achieve competitive, less energyintensive acid gas removal alternatives.

Most physical CO₂ adsorbents such as 13× zeolite, activated carbons,periodic mesoporous silica and metal-organic frameworks (MOFs) require alarge pressure and/or temperature gradient between the adsorption anddesorption stages to enable both efficient adsorption performances andnear complete desorption of CO₂. Moreover, they exhibit relatively lowselectivity toward CO₂, generally low tolerance to water vapour in thegas feed, and their CO₂ separation performance decreases drastically byincreasing the temperature.

Incorporation of amine groups onto large surface area porous solids viadirect synthesis, impregnation, surface polymerization, surface graftingor co-condensation as a promising approach for CO₂ capture has gainedprominence in recent years. When properly designed, such materialsexhibit high adsorption capacity, fast CO₂ adsorption and desorption andlow energy recycling requirement.

In addition to the above-mentioned attributes, amine-containingmaterials are tolerant to the occurrence of moisture in the feed.Actually, CO₂ adsorption is enhanced by the presence of moisture in thefeed (Serna-Guerrero et al. 2008). The reason lies within the nature ofthe amine-CO₂ interactions. Under dry adsorption conditions, surfaceamine groups interact with CO₂ to form carbamate with a stoichiometricCO₂/N ratio of 0.5, whereas under proper humidity conditions,bicarbonate with a stoichiometric ratio CO₂/N=1 may be formed(Serna-Guerrero et al. 2008). In contrast, in the presence of otheradsorbents such as zeolites and activated carbons, CO₂ adsorption isinhibited by moisture because of unfavourable competition.

However, despite the large number of contributions devoted to CO₂adsorption over amine-containing materials (Choi et al. 2009), anddespite the utmost importance of the long term stability of suchmaterials, no studies addressed the issue of adsorbent stability throughextensive recycling.

The lifetime of adsorbents, which determines the frequency of theirreplacement, is a critical parameter of equal importance as CO₂adsorption capacity, selectivity and kinetics, having a direct impact onthe economics of any commercial scale operation.

This background information is provided for the purpose of making knowninformation believed by the applicant to be of possible relevance to thepresent invention. No admission is necessarily intended, nor should beconstrued, that any of the preceding information constitutes prior artagainst the present invention.

SUMMARY

An object of the present invention is to provide a method and a systemfor stabilizing amine-containing adsorbents, for example, for use insteady state operation of amine-containing adsorbents for the purpose ofCO₂ capture and removal.

In accordance with one aspect, there is provided a method of stabilizingan amine-containing CO₂ adsorbent comprising contacting saidamine-containing CO₂ adsorbent with one or more humid gas streams. Thismethod can be employed in a process for selectively removing orrecovering CO₂ from a gaseous stream or atmosphere containing CO₂.

It has now been found that the stability of amine-containing CO₂adsorbents used in a system for removal of CO₂ is enhanced whenadsorption and desorption stages are carried out in the presence ofmoisture-containing feed and purge gases. Furthermore, the CO₂adsorption capacity of triamine-grafted mesoporous silica demonstrates agradual decrease upon adsorption-desorption cycling under mildconditions. In contrast, the stability of triamine-grafted mesoporoussilica drastically improves in the presence of moisture-containinggases. The loss in stability of amine-containing adsorbent material hasbeen found to be linked to the formation and accumulation of urea groupseven under mild, but dry conditions, while the formation of urea groupis inhibited in the presence of moisture.

In accordance with another aspect, there is provided a method forregeneration of a deactivated amine-containing CO₂ adsorbent comprisinghydrolyzing urea groups in said deactivated adsorbent at hightemperature in the presence of moisture.

In accordance with another aspect, there is provided a system forremoval of CO₂ from a gaseous stream comprising: (a) one or more sorbentbeds comprising an amine-containing adsorbent; (b) means for controllinga CO₂-containing feed gas flow through said one or more sorbent beds;(c) means for controlling a purge gas flow through said one or moresorbent beds; (d) means for controlling the relative humidity of saidfeed gas, said purge gas or both said feed gas and said purge gas. Thesystem for CO₂ removal can include means of removing the CO₂ from thesorbent bed by pressure-swing (PS), vacuum-swing (VS), temperature-swing(TS) regeneration modes or a combination thereof in fixed, moving orfluidized beds; a means of adjusting the relative humidity of thegaseous stream used for adsorption and/or desorption, and a means ofadjusting the level of hydration of the sorbent bed during adsorptionand/or desorption.

In accordance with another aspect, there is provided a system forremoving or recovering CO₂ from an gaseous stream or atmospherecontaining said CO₂ using amine-containing CO₂ adsorbents with enhancedstability in terms of CO₂ capture performance over hundreds ofadsorption-desorption cycles in the presence of moisture-containing feedgas and/or moisture-containing purge gas in the range of about 0.1% to100% relative humidity (RH).

In accordance with another aspect of the invention, there is provided asystem for removing or recovering CO₂ from a gaseous stream oratmosphere containing said CO₂ using different selectiveamine-containing CO₂ adsorbent where the purge gas may bemoisture-containing non-adsorbing gas, moisture-containing CO₂ or steam.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 graphically depicts dynamic CO₂ adsorption over a number of CO₂adsorption-desorption cycles on mono amine and triamine graftedpore-expanded MCM-41 (MONO and TRI, respectively) andpolyethyleneimine-impregnated pore-expanded MCM-41 (PEI) carried outunder various conditions using dry feed and purge gases.

FIG. 2 graphically depicts dynamic CO₂ adsorption over a number of CO₂adsorption-desorption cycles on mono amine and triamine graftedpore-expanded MCM-41 (MONO and TRI, respectively) andpolyethyleneimine-impregnated pore-expanded MCM-41 (PEI) carried outunder various conditions using wet feed and purge gases.

FIG. 3 depicts ¹³C CP MAS NMR for a series of MONO samples and UREA(bis(trimethoxysilylpropyl)urea-grafted MCM-41).

FIG. 4 depicts in situ DRIFT spectra for fresh MONO (MONO-Fresh), MONOafter 40 cycles under dry (MONO-105/105-d) and humid (MONO-105/105-h)conditions and for fresh UREA (UREA-Fresh).

FIG. 5 depicts CO₂ adsorption-desorption cycles on TRI-PE-MCM-41 in dryand humid (20° C. as dew point, 7.5% RH) conditions using adsorption anddesorption at 70° C.

FIG. 6 depicts ¹³C NMR spectra for fresh MONO (MONO-Fresh), MONO aftercycling in dry (MONO-105/105-d) condition, and MONO-105/105-d afterhydrolysis (MONO-105/105-d-hydr).

FIG. 7 depicts ¹³C NMR spectra for fresh UREA (UREA-Fresh), UREA-Freshafter hydrolysis (UREA-Fresh-hydr) and MONO-Fresh.

FIG. 8 depicts DRIFT spectra for fresh UREA (UREA-Fresh), UREA-Freshafter hydrolysis (UREA-Fresh-hydr) and MONO-Fresh.

DETAILED DESCRIPTION OF THE INVENTION

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs.

As used in the specification and claims, the singular forms “a”, “an”and “the” include plural references unless the context clearly dictatesotherwise.

The term “comprising” as used herein will be understood to mean that thelist following is non-exhaustive and may or may not include any otheradditional suitable items, for example one or more further feature(s),component(s) and/or ingredient(s) as appropriate.

It has now been shown that during CO₂ adsorption-desorption cyclingunder dry conditions, amine-containing adsorbent materials ultimatelydeactivate, even under mild conditions, through formation of urea groupson and/or in the adsorbent materials. In order to minimize deactivation,the present invention provides a method to enhance the stability of suchamine-containing materials. The present invention further provides amethod for regeneration of the deactivated adsorbent materials.

The present application provides a method of stabilizingamine-containing materials used as CO₂ adsorbents. The amine-containingadsorbents retain a stable adsorption capacity when used under humidconditions. Accordingly, the present invention provides a method ofstabilizing an amine-containing CO₂ adsorbent that includes the step ofcontacting the adsorbent with a gas containing CO₂ and possibly otheracid gases, having a relative humidity of at least about 0.1%, and/orthe step of regenerating the adsorbent in the presence of a purge gaswith a relative humidity from about 0.1% to about 100%.

Amine-Containing Adsorbent

The amine-containing adsorbents useful in the method and systemdescribed herein are large surface area porous solids that includeamines that are accessible to the adsorbate during an adsorption processto remove CO₂ from a gas.

Generally, there are three major classes of amine-containing adsorbentssuitable for CO₂ removal, namely (i) materials with surface-anchoredamine species prepared by aminosilane grafting or cocondensation orin-situ surface polymerization of reactive amines such as aziridine(Hicks et al., 2008), (ii) amine-containing species such as ethanolamine(Franchi et al., 2005) or polyethyleneimine (Yue et al. 2008, Ma et al.2009), impregnated onto materials with high surface area such as carbonor mesoporous silica, and (iii) amine-containing MOFs (Couck et al.2009; Vaidhyanathan et al. 2009; Arstad et al. 2008). A suitableadsorbent can be prepared using various methods, including those alreadyknown, in order to obtain CO₂ adsorbents having varying capacities andrates of adsorption depending on the potential use of the material.

Non-limiting examples of suitable amine-containing adsorbents aremonoamine and triamine-grafted pore-expanded MCM-41 mesoporous silica,polyethyleneimine-impregnated PE-MCM-41, and amine-functionalized MOFs.

The amines used in the preparation of the adsorbent must exhibitsufficient basicity to allow for efficient reaction with CO_(2,) and/orother acidic gases to be adsorbed. In addition a high N/C ratio can bebeneficial to maximising the concentration of amine groups within theadsorbent. In order to allow effective regeneration of the adsorbent,the adsorbent should be thermally stable during the desorption process.

The amines may be primary amines, secondary amines, tertiary amines,mixed amines or any combination thereof. As shown in the followingsection, amines can be introduced via different routes including (i)grafting or co-condensation using amine-containing trialkoxy- ortrichlorosilanes, (ii) adsorption, (iv) impregnation, (iv) surfacepolymerization of amine-containing monomers, (v) synthesis orpost-synthesis pore expansion using amines, (vi) reaction with frameworkor with pending reactive groups, and (vii) self assembly with silica ororganosilica precursors using amphiphile amines. The means by whichamines are introduced to solid supports will change depending on thetype of support material used, the desired adsorbent configuration, thenature of the amine, etc.

Selection of the specific amine or amines to be used in the preparationof the adsorbent of the present invention will depend on theconfiguration of the adsorbent and on the application for which theadsorbent is intended. For example, in cases where a high adsorptivecapacity is not critical, the amine or amines will be selected keepingin mind characteristics such as high regeneration ability, low cost andready availability rather than maximum reactivity. In general, primaryand secondary amines are more reactive with acidic gases than tertiaryamines. Similarly, primary amines are generally more reactive thansecondary amines. Further, the configuration of the adsorbent may imposelimitations on the nature of the amine that can be used.

CO₂ Adsorption Method and System

The present application further provides a method and a system forremoving CO₂ exclusively or combined with other acid gases, such as H₂S,from a gaseous stream containing CO₂ and possibly one or more other acidgases. For simplicity, the following discussion specifically refers toCO₂ only, as the acid gas, however, it should be understood that otheracid gases will also be removed, if they are available in the gaseousstream. Further provided is a system and process for CO₂ adsorption thatinclude a means for, or the step of, contacting an amine-containingadsorbent alternatively with feed and purge gases at atmospheric orsub-atmospheric pressure, that have a relative humidity of at leastabout 0.1%, or as high as about 100%.

According to the presently described method and system, anamine-containing adsorbent is employed in a sorbent bed for use in acyclic adsorption-desorption process carried out under humid conditions.In accordance with one embodiment, the CO₂ adsorption and desorption arecarried out using feed and/or purge gases having a relative humidity ofat least about 0.1%. In accordance with another embodiment, the feedand/or purge gases have a relative humidity of from about 0.1% to about100%. In accordance with another embodiment, the purge gas may be a wetinert gas, a wet non-adsorbing gas, wet CO₂ or steam. In accordance withanother embodiment, the total pressure of the purge gas is varyingbetween 0.05 bar and 1 bar.

To apply an amine-containing adsorbent to such a cyclicadsorption-desorption process, the adsorbent must be formed into astable, mechanically strong form. These forms may include, but are notlimited to, powder forms, pellet forms, monolithic structures or foams.In the case of pellet forms, the adsorbent is mixed with a suitableinert or active secondary material as a binder. Criteria for selecting asuitable binder can include (i) achieving pellets or extrudates withminimum amount of binder; (ii) enhanced mechanical stability; (iii)preservation of adsorbent porosity and accessibility of adsorptionsites; and (iv) affordability. For example, siloxanes and siloxanederivatives can be employed to form structured pellets, eitherextrudates or spheres, using the appropriate weight percentage ofadditive. The selection of the appropriate form and, if necessary,additive, is based on the application of the adsorbent and the type ofequipment used in the CO₂ removal process. The selection and manufactureof the adsorbent form is well within the ordinary abilities of a workerskilled in the art.

Once the adsorbent form is selected and manufactured, it is used in asorbent bed where a gaseous stream containing CO_(2,) and having arelative humidity of at least about 0.1%, contacts the adsorbent. TheCO₂ and amine or CO_(2,) amine and water chemically react to form acomplex, thereby removing the CO₂ from the gaseous stream.

According to a specific embodiment, once the adsorbent is loaded withCO₂ to a satisfactory level, for example, when greater than 80% of theamine has been converted to the amine complex, or at a designated cycletime, the sorbent bed can be regenerated. Regeneration comprises ceasingthe flow of the gaseous stream through the bed and desorbing theadsorbed CO₂ at atmospheric or sub-atmospheric pressure. The endothermicdesorption reaction is accomplished by thermal and/or pressure gradientmeans and by the use of a purge gas, or any combination thereof. Duringthis step, the amine complex is dissociated, CO₂ is removed and theamine is freed and ready for re-use.

Various means can be incorporated in the system of the present inventionto produce a relative humidity of at least about 0.1% in the feed andpurge gases. In accordance with one embodiment, the system includes awater humidifier through which the feed and the purge gases used foradsorption and desorption are contacted with water at an appropriatetemperature to achieve the desired relative humidity at the desiredadsorption or desorption temperature. Other means by which the humidityof gases can be adjusted include, but are not limited to, heat andmoisture exchangers and ultrasonic nebulizers.

It is understood that a suitable adsorbent is not necessarily limited touse for the exclusive removal of CO₂ from a gaseous stream. Rather theadsorbent can be used for the removal of additional acid gases, if theyoccur in the CO₂-containing gaseous feed.

In one embodiment, use of the adsorbent to remove CO_(2,) with orwithout other acid gases, can comprise utilising two or more sorbentbeds operating cyclically such that the first bed is in the adsorptioncycle while the second bed is in the desorption cycle. Such a systemcomprises two or more sorbent beds and computer or manually controlledvalves and pumps allowing for continuous removal of CO₂ (and possiblyother acid gases) from the gaseous stream. In the adsorption cycle, anexothermic reaction occurs between CO₂ in the gaseous stream, which isflowing through the adsorbent, and the amine present in the adsorbent,thereby adsorbing the CO₂ and forming a CO₂-amine complex. This complexcan also be a combination of amine, CO₂ and water. In one embodiment ofthe present invention, the heat produced during the adsorption processin the first bed can be transferred via a heat exchanger to the secondbed to drive the endothermic desorption of the adsorbed CO₂ and watersimultaneously occurring therein. Alternatively, the desorption processcan be effected through thermal and/or pressure gradient meansindependent of the adsorption process. Highly stable adsorbent isobtained when the gas feed used for adsorption and/or the purge gas usedfor desorption contain water vapour at a relative humidity of at leastca. 0.1% up to ca. 100%.

Among the different possibilities, moisture-containing CO₂ or steam canbe used as purge gas to desorb CO_(2.) In both cases, pure CO₂ may beseparated, and if need be, pressurized and stored.

To gain a better understanding of the invention described herein, thefollowing examples are set forth. It should be understood that theseexamples are for illustrative purposes only. Therefore, they should notlimit the scope of this invention in any way.

EXAMPLES Example 1 Adsorption-Desorption Cycling of Amine-Containing CO₂Adsorbents in the Presence and Absence of Moisture

The loss of stability of amine-containing adsorbents through extensiveCO₂ adsorption-desorption cycling under dry conditions, and thestabilization of amine-containing adsorbents through operation underhumid conditions were investigated using triamine (TRI-PE-MCM-41) and,monamine (MONO-PE-MCM-41)-grafted pore-expanded MCM-41 mesoporous silicaand polyethyleneimine-impregnated PE-MCM-41 (PEI-PE-MCM-41).

Materials Preparation of Amine-Modified Materials

TRI-PE-MCM-41 and MONO-PE-MCM-41 were prepared following the proceduresdescribed elsewhere (Sayari and Harlick 2010 and Serna-Guerrero et al.2008, respectively. Briefly, MCM-41 type silica was produced at 100° C.in the presence of cetyltrimethylammonium bromide in basic conditionsusing tetramethylammonium hydroxide. Subsequently, the pore size ofMCM-41 was increased by hydrothermal restructuring at 120° C. in thepresence of an aqueous suspension of dimethyldecylamine [Sayari et al.,1998]. The surfactant template and pore expander agent were removed bycalcination in nitrogen, then in air at 550° C., providing PE-MCM-41.The amine functional groups were incorporated onto PE-MCM-41 by graftingin toluene at 85° C. in a 250 mL glass reactor. First, 0.3 ml of waterper g of silica was added to a PE-MCM-41 silica suspension in toluene,followed by addition of monoamine or triamine-containing silane (3 mlper g of silica). Grafting proceeded for 16 h, and then the product wasfiltered and washed with toluene and pentane.

Polyethyleneimine-containing material was prepared by dispersing PEI(molecular weight=324) on PE-MCM-41 as follows. An amount of 1 g of PEIwas dispersed in methanol at room temperature followed by the additionof 1 g PE-MCM-41 to the mixture under continuous stirring. The mixturewas stirred for 30 min and the slurry was dried overnight under ambientair.

Two amine-containing MOFs, namely amino-MIL-53 andZn-aminotriazolato-oxalate were also prepared according to the methodsset out in Ahnfeldt et al. 2009 and Vaidhyanathan et al. 2009,respectively.

Table 1 provides the structural properties and amine content of thematerials, referred to as MONO, TRI and PEI.

TABLE 1 Characteristics of amine-containing CO₂ adsorbents Amine- Porecontaining Surface area volume Pore size Amine Amine material S_(BET)(m²/g) V_(p) (cm³/g) D_(p) (nm) loading type^(a) MONO 420 0.65 7.2 4.3mmol/g I TRI 367 0.87 9.4 7.9 mmol/g I, II PEI 4.5 * * 50 wt %   I, II,III ^(a)I: Primary; II: Secondary; III Tertiary.

2. Preparation of Mesoporous Silica Containing Urea Groups

The urea-containing species was incorporated onto PE-MCM-41 by grafting(bis(trimethoxysilylpropyl)urea in toluene at 85° C. using a 250 mlglass reactor. First, 0.3 ml of water per gram of silica was added to aPE-MCM-41 silica suspension in toluene, followed by addition of thesilane (1.5 ml per gram of silica). Grafting proceeded for 16 h, andthen the product was filtered and washed with toluene and pentane. Theobtained material was labelled as UREA. The characteristics of the UREAsample were as follows: Urea loading: 4.3 mmol/g; surface area: 546m²/g; pore volume: 0.8 cm³/g and pore diameter: 6.1 nm.

Cyclic CO₂ Adsorption-Desorption Studies

The cyclic CO₂ adsorption-desorption experiments were carried out usinga Rubotherm magnetic microbalance (Belmabkhout and Sayari, 2009). Theprocedure was as follows: the sample was first activated in the presenceof flowing UHP nitrogen at 50 mL/min for 30 min at a specifiedtemperature (see below) under atmospheric pressure. Subsequently, thesample was cooled down to the adsorption temperature under isobariccondition and the feed gas was switched to pure CO₂ at 50 mL/min. Theeffective adsorption capacity (non-equilibrium) was assumed to be theweight gain of the sample after 30 min exposure. This procedure wasrepeated over tens or hundreds cycles (i.e., regeneration and CO₂adsorption).

All the CO₂ adsorption-desorption cycling experiments reported hereinwere carried out at atmospheric pressure. The gas feed (pure CO₂) andthe purge gas (pure N₂) used for desorption were either dry or containeda level of moisture corresponding to the equilibrium vapour pressure ofwater at 20° C. The actual relative humidity (RH) depended on thetemperature of the gas stream, varying from 100% at 20° C. to 0.4% at150° C. Under such conditions, the triamine-grafted material undergoingadsorption at X° C. and desorption at Y° C. under dry conditions will bereferred to as TRI-X/Y-d. When the cycling experiment is carried outusing moisture-containing gas streams, the material will be designatedas TRI-X/Y-h. The same nomenclature applies to the other adsorbents.

FIG. 1 shows the dynamic CO₂ adsorption uptake over a number ofadsorption-desorption cycles carried out at different conditions, in thepresence of different adsorbents. As seen, in all cases, the materialsdeactivate at a rate that is dependent on the severity of the cyclingconditions and on the nature of the adsorbent. The percentage loss ofcapacity was 14% for TRI-50/120-d over 40 cycles and as high as 41% forPEI-105/105-d for only 22 cycles. Under the same cycling conditions,adsorbents with grafted amines, MONO-105/105-d and TRI-105/105-d,deactivated by 45% over 40 cycles.

FIG. 2 shows the CO₂ adsorption capacity in the same series ofexperiments as in FIG. 1, except that the feed and the purge gases usedfor adsorption and desorption were bubbled through a water saturatormaintained at 20° C. As seen, dramatic improvement in the stability ofthe materials was obtained through the use of humid conditions. In allcases, the adsorption capacity was stable throughout all cycles. Thus,this data demonstrates the enhanced stability of amine-containing CO₂adsorbent in the presence of moisture-containing gases, in comparison tothe stability in the presence of the same gases without added moisture.

¹³C CP MAS NMR data for a series of MONO samples are shown in FIG. 3. Inaddition to signals associated with the propyl carbon chain below 50ppm, the fresh sample exhibited a signal at 165 ppm attributable tocarbamate formed via adsorption of atmospheric CO₂. The MONO-105/105-d,which lost ca. 45% of its adsorption capacity upon CO₂adsorption-desorption cycling under dry conditions (FIG. 1), exhibitedan additional peak at 160 ppm. Interestingly, this peak did not occur inMONO-105/105-h, which underwent the same CO₂ cycling procedure, but inhumid conditions, i.e., without loss of adsorption capacity. Thus,without wishing to be bound by theory, it was inferred that such speciesis responsible for the gradual loss of CO₂ adsorption capacity. Tounambiguously identify this species, it was compared to the signal ofthe C═O group of bis(trimethoxysilylpropyl)urea-grafted MCM-41, referredto as UREA. Based on the data shown in FIG. 3, it is clear that thegradual deactivation of the adsorbents is due to the progressiveformation of urea groups.

The contention that the formation of stable urea groups, favoured underdry conditions, is responsible for the deactivation of theamine-containing CO₂ adsorbents was further substantiated by in situdiffuse reflectance infrared Fourier transform (DRIFT). As shown in FIG.4, as the number of CO₂ adsorption-desorption cycles increased, twobands at 1658 and 1560 cm⁻¹ developed. Comparison with the DRIFTspectrum of the UREA sample shows clearly that the two new bands areassociated with the occurrence of urea groups.

The occurrence of urea groups takes place even under mild conditions,albeit at much slower rate. FIG. 5 shows that under mild, but dryconditions, TRI-70/70-d which seems to be stable over dozens of cycles,actually deactivates gradually over hundreds of cycles to reach 15% lossafter ca. 750 cycles. As in the experiments described earlier,humidification of the gas streams led to a stable material over 700cycles.

Scheme 1A shows the relationship between amine, CO_(2,) carbamate andurea species during CO₂ adsorption-desorption under dry conditions. Theurea groups may form by direct interaction between amine species and CO₂at high temperature. Drage et al., 2008 carried out temperatureprogrammed adsorption of CO₂ on polyethyleneimine loaded silica underflowing CO_(2.) As the temperature increased, the weight decreased dueto the carbamate decomposition. Then, starting at ca. 135° C., a weightincrease corresponding to a secondary reaction between CO₂ and aminegroups was associated with the formation of urea at high temperature.The experiments described herein demonstrate that under repeated CO₂adsorption-desorption cycling, provided that the gas streams are dry,urea groups will form incrementally during adsorption and/or desorption(via carbamate decomposition) even under much milder conditions.Accumulation of heat-resistant urea groups leads ultimately to materialdeactivation. In contrast, Scheme 1B shows that if moisture-containinggases are used, the formation of urea is strongly inhibited. Indeed,even at a relative humidity as low as 0.4%, as in the case ofTRI-70/150-h (FIG. 2), the material did not deactivate.

Deactivation via formation of urea groups can be reversed via hydrolysisof such species, while preserving the integrity of the material. Heatingdeactivated MONO-105/105-d at 200° C. under a flow of nitrogencontaining as little as 0.15% RH (dew point at 20° C.) for 24 h restoredthe grafted propylamine completely, as evidenced by ¹³C NMR (FIG. 6) andCO₂ adsorption measurements. Nitrogen adsorption measurements at 77 Kshowed that the structural characteristics (Table 1) of the materialwere also preserved. Thus, the present invention further provides amethod of recovering the adsorption properties for deactivatedamine-containing CO₂ adsorbents. The method comprises the step ofhydrolyzing urea groups formed during the adsorption phase.

Example 2 Preparation of Amine-Containing CO₂ Adsorbents Via Hydrolysisof Bis(Trialkoxysilylalkyl)Urea-Grafted Materials

As described above, the UREA sample was hydrolyzed under similarconditions (flowing

N₂, 200° C., 0.15% RH, 24 h) to generate a propylamine-graftedPE-MCM-41, This is clearly shown by ¹³C NMR (FIG. 7), and DRIFT (FIG.8). The material thus obtained proved to be an excellent CO₂ adsorbentwith an equilibrium capacity of 1.3 mmol/g (5.7 wt %) at roomtemperature and 1 atm in the presence of 5% CO₂ in N₂. This correspondsto a CO₂ adsorption efficiency of CO₂/N=0.49.

REFERENCES

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All publications, patents and patent applications mentioned in thisSpecification are indicative of the level of skill of those skilled inthe art to which this invention pertains and are herein incorporated byreference to the same extent as if each individual publication, patent,or patent applications was specifically and individually indicated to beincorporated by reference.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. A method of stabilizing an amine-containing CO₂ adsorbent overmultiple adsorption-desorption cycles comprising contacting saidamine-containing CO₂ adsorbent with one or more humid gas streams tostabilize the adsorbent, wherein the method comprises an adsorption stepof contacting said amine-containing CO₂ adsorbent with said one or morehumid gas streams, and subsequently a desorption step of contacting saidCO₂-loaded amine-containing adsorbent with said one or more humid or oneor more dry gas streams, wherein said contacting steps are repeatedduring said multiple adsorption-desorption cycles of said system forremoval of CO₂.
 2. The method according to claim 1, wherein said one ormore humid gas streams in said adsorption step is a humid CO₂-containingfeed gas stream, or a humidified CO₂-containing feed gas stream.
 3. Themethod according to claim 1, wherein said one or more humid or dry gasstreams in said desorption step is a humid purge gas stream, ahumidified purge gas stream or a dry purge gas stream.
 4. The methodaccording to claim 1, wherein said amine-containing CO₂ adsorbent iswithin a system for removal of CO₂ from a humid or humidifiedCO₂-containing feed gas stream, and wherein said method comprisescontacting said amine-containing CO₂ adsorbent with said CO₂-containinghumid or humidified feed gas stream according to claim 2, andsubsequently contacting said CO₂-loaded amine-containing adsorbent witha humid or dry purge gas stream according to claim
 3. 5. The methodaccording to claim 4, wherein the purge gas stream is a humid or dryinert gas, a humid or dry non-adsorbing gas, humid or dry CO₂-rich orsteam.
 6. The method according to claim 1, wherein the relative humidityof humid or humidified gas streams is from about 0.1% to about 100%. 7.The method according to claim 1, wherein said amine-containing CO₂adsorbent is comprised, but not limited to, amines or polyamines graftedon inorganic matrices, organic matrices, hybrid matrices, organometallicmatrices or a combination thereof, amines or polyamines impregnated oninorganic matrices, organic matrices, hybrid matrices, organometallicmatrices or a combination thereof.